The present invention relates to a scanning charged particle microscope to irradiate a charged particle beam narrowed down onto a sample, and to obtain a microscopic image of the sample on a display unit such as a CRT etc., and especially to a facilitation method of a focusing working and an astigmatism correction working of the microscope.
In a charged particle microscope such as a scanning electron microscope (SEM) and a scanning ion microscope (SIM), a sample face is scanned with the charged particle beam such as the electron beam or the ion beam which is narrowed down, microscopic image is displayed on the CRT by using signals of second particles such as second electrons occured from the sample as brightness modulation signal for the CRT. Focal distance adjusting of the microscopic image is performed usually by regulating a focal distance of an objective lens which met the sample. In the scanning electron microscope of late years, this focal point adjusting working is automatically performed by using a differentiated signal of the second particle signal relating to the the scanning distance thereof. When an operator pushes button, the for focal distance where the differential signal becomes to be the greatest, is obtained, an operating condition of the objective lens changes automatically. The focal point adjusting method that used this differential signal still has an incomplete part. Because the strength of the differential signal is related to contrast (a concentration change of design) of the microscopic image in addition to fineness of the beam, value of the differential signal becomes small and reliability of this automatic focusing method becomes low in the microscopic image being small contrast. Because the contrast in a high magnification image is generally small, this automatic focusing method may not be operated well, when the focal distance in the high magnification image is going to be put together. In addition, this automatic focusing method has a weak point to adjust a focal distance for a sample with a unclear figure.
On the other hand, in transmission type electron microscope (TEM) that principle of the microscopy is basically different from SEM, there is a focal distance adjusting apparatus called Wobbler apparatus, and it has been used as an apparatus being convenient and having high reliability for a long time. In Wobbler apparatus, the electron beam for irradiating the sample is inclined to an optical axis thereof in turn. Operator may observe the microscopic image of the sample magnified on a fluorescence version. If the microscope is out of focus, the image on the fluorescent screen is swung from side to side, and the operator regulates a focal distance of the objective lens so as not to swing the image.
The differential signal in the automatic focusing method of the scanning electron microscope, is extracted as an information of every picture elements of the microscopic image. On the other hand, the swing of the image in Wobbler method of TEM is recognized by extracting and using an information of whole image. The detection of the swing of the image may be executed under extremely high signal to noise ratio and high reliability comparing with the differential signal detection, and the focal distance adjusting is performed without any problem for a low contrast sample. In late years, this principle becomes to be applied in the world of TEM. For example, such apparatus are described in Japanese Patent Leid-open Nos. 59-112556(1984) and 55-46447(1980). The present invention plans to apply such principle in SEM and SIM. If the automatic focusing may be performed by using only two pieces of microscopic image by irradiating the electron beam with different angles to the sample as in the automatic focusing of TEM Wobbler method, such principle is easily used in SEM and SIM. However, if particle beam in conventional SEM/SIM is irradiated to the sample by inclining it as in Wobbler method, only a location difference occurs in proportion to angle of the inclination and any information about the out of focus is not provided in the microscopic image. That is, even if it is in focus, the movement of the microscopic image occurs in the same way and extent of the out of focus is not measured.
The first object of the present invention is to provide an apparatus which can focus by recognizing an amount of the swing or the move of the microscopic image in the scanning charged particle microscope. The second object of the present invention is to provide an apparatus which can easily correct astigmatism of the scanning charged particle microscope. The third object of the present invention is to provide an apparatus which can focus and can easily correct the astigmatism in a short time, and remarkably reduce damage and contamination of the sample which the beam irradiation causes.
In order to achieve the object mentioned above, in a scanning charged particle microscope of the present invention, an optical system is constituted so as to provide a crossover of the charged particle beam between a charged particle gun and a beam scanning device, and provided a beam deflector so as to deflect the beam at a crossover point as a supporting point.
That is to say, the scanning charged particle microscope in the present invention has a charged particle gun for generating a charged particle beam, an objective lens for irradiating said charged particle beam narrowed down onto a sample, and a scanner for scanning on said sample with said charged particle beam, and the scanning charged particle microscope in the present invention is characterized by comprising a crossover of said charged particle beam being provided between said said charged particle gun and said scanner, and a deflector for deflecting said charged particle beam at said crossover as a supporting point. A beam deflection motion performed by the deflector can be done with a cyclic mode in time repeatedly.
In addition, an image memory to respectively memorize plural pieces of the microscopic images obtained by irradiating the charged particle beams with different deflection-angles, and an operational unit for calculating an amount of figure difference between those microscopic images may be provided in the present invention. Then, it is desirable to have a controller to calculate a focal distance correction amount being necessary for adjusting focal distance of the microscopic image from the amount of the figure difference between plural pieces of the microscopic images, and to set the focal distance of the objective lens to be a value revised by said focal distance correction amount.
In addition, when the charged particle beam is deflected so as to go around a conical surface along a slant line of a circular cone having the crossover as a peak thereof, the correction of the astigmatism can be done based on the information of position moving of the microscopic image by deflecting the beam. That is, an automatic compensation of the astigmatism can be performed by providing an image memory to respectively memorize plural pieces of the microscopic images obtained by using the charged particle beam with different deflection-angle, an operation means for obtaining an ellipse fitting a moving locus of plural pieces of the microscopic images memorized in the image memory, an operation means to calculate a size and a direction of the astigmatism from a length and a leaning angle of major axis and minor axis of an ellipse, and a correction device of the astigmatism driven based on these information.
In the scanning charged particle microscope having the charged particle gun for generating the charged particle beam, the objective lens to irradiate the charged particle beam narrowed down on the sample and the scanner which scans the charged particle beam on the sample in two dimensional, a focal distance adjusting method of the scanning charged particle microscope in the present invention is characterized by providing a crossover of said charged particle beam being provided between said said charged particle gun and said scanner and by regulating the objective lens so as to make a moving amount of the microscopic image minimum when deflecting the charged particle beam at the crossover point as the supporting point.
In the focal distance adjusting method of the scanning charged particle microscope having the charged particle gun for generating the charged particle beam, the objective lens to irradiate the charged particle beam narrowed down on the sample and the scanner which scans the charged particle beam on the sample in two dimentional, the scanner which scans the charged particle beam on the sample in two dimentional and the correction device of the astigmatism, the focal distance adjusting method in the present invention is characterized by providing a crossover of said charged particle beam being provided between said said charged particle gun and said scanner and by regulating the correction device of the astigmatism so as to move the microscopic image to draw a perfect circle when the charged particle beam is deflected so as to go around a conical surface along a slant line of a circular cone having the crossover as a peak thereof.
The scanning charged particle microscope in the present invention can be applied to an elemental analysis of a sample minute part, processing of a minute sample part, and length measurement of the minute sample part. According to the present invention, the focal distance adjusting and the astigmatism correction of the scanning charged particle microscope becomes possible to be easily done, and sample contamination and damage to occur while those working can be reduced.